Poor Precision

Poor precision is also indicated by poor or high relative standard deviation or a noisy signal.

There are a multitude of potential causes for an instrument exhibiting poor precision. These include, but are not limited to:

• Identifying the problem
• V-groove nebulizer
• Concentric glass nebulizer
• Torch
• Contamination in spray chamber
• Pump/pump tubing
• Air leak
• Instrument calibration
• Aqueous analysis after non-aqueous analysis
• Partially soluble salts
• High salt content

Identifying the Problem

Poor or high relative standard deviations are generally caused by poor functioning of the sample introduction system. Poor precision can also be a symptom of low sensitivity. Make sure intensities are correct by running the Instrument Performance test and then comparing the observed Resolution, Sensitivity, and Precision values to values listed in the test before attempting to resolve precision issues. If results are comparable it suggests the problem may be related to the method (application) and may not be a fault of the instrument. If any of the results vary by ~20% then rerun the test with a known good SeaSpray Nebulizer and Cyclonic spray chamber to eliminate this as a possible cause. Poor comparable Precision numbers will verify a problem exists. Check which wavelengths are poor and by how much. Compare to installation or past installation qualification numbers. The table below will give some direction as to failure mode and possible solution.

Precision is a measure of signal repeatability over multiple replicates (typically 10 replicates for instrument performance test) and expressed as %RSD. A single flyer (high or low reading) can cause this test to fail.

If you observe poor or high relative standard deviations you need to determine if it is being caused by drift (for example, steady increase or decrease of replicate signals), noise, or a flyer.

• Drift or noise — To check on possible signal drift or noise, use the time scan facility to monitor the analytical signal while aspirating a wavelength calibration solution or 5 ppm Mn solution. Instructions on how to perform this procedure are here.
• Flyers — Monitor replicates and check for flyers (especially the first reading). If this reading is low it suggests the uptake delay time is too short for the length of tubing and needs to be increased.

Failure Mode (Precision)	Possible cause	Possible cause (Next level)
All or most wavelengths have poor precision	Sample introduction components contaminated	Spray chamber walls dirty (water beading) Nebulizer partially blocked (uneven spray) Nebulizer tip damaged Torch injector tube contaminated
	Inconsistent sample delivery	Pump tube pressure incorrect Pulsations due to low pump speed Crimped or damaged pump tube Uptake delay time too short for sample uptake tubing length (first reading low)
	Optimization of parameters	Nebulizer flow Viewing height (radial or SVDV instruments only)
	Wavelengths off peak	Instrument warm up - let the instrument warm up for 20 minutes Instrument calibration
	Running partially soluble (salts)	Partial blocking of nebulizer and injector tube
	High salts	Partial Blocking of nebulizer and injector tube
	Previously running organics	Contaminated spray chamber, nebulizer, torch injector
	Unstable plasma	Contaminated argon Air leak - check all connections and fittings
Some wavelengths have poor precision	Wavelengths off peak	Instrument warm up - let the instrument warm up for 20 minutes Instrument calibration

 

V-groove Nebulizer

Check the nebulizer for blockage by aspirating distilled water through the sample introduction system and carefully removing the nebulizer from the spray chamber. Hold the nebulizer in its normal operating position and observe the spray. The spray should be even, without spurting. If an even spray cannot be achieved by adjusting the peristaltic pump, clean the nebulizer.

There will be some degree of pulsing visible which is acceptable.

Concentric Glass Nebulizer

• Inspect the nebulizer tip with a magnifying glass. If the tip is damaged, replace the nebulizer.
• Check the nebulizer for a blockage.
• Aspirate distilled water through the sample introduction system, carefully remove the nebulizer from the spray chamber and observe the spray emanating from the nebulizer tip. The sample should spray evenly, without spurting, from the tip of the nebulizer, depending on the pump speed. If the spray pattern is irregular, the nebulizer orifice may be blocked in which case you will need to clean the nebulizer.

There will be some degree of pulsing visible which is acceptable.

Check the capillary within the nebulizer. An internal blockage may require an excessively high nebulizer flow to nebulize a solution.

Tip A leaking sample inlet capillary often signifies a blockage in the nebulizer capillary.

Torch

Check the torch for deposits that may interfere with the instrument optics. A clean injector tube is essential. Clean the torch if necessary.

If the injector tube of the torch is broken or worn, replace the torch.

Contamination in Spray Chamber

Disconnect the spray chamber from the sample introduction system and clean the spray chamber before replacing it.

Pump/Pump Tubing

To check the operation of the pump and pump tubing, dip the sample tubing in and out of the solution and watch the sample uptake along the capillary. If the liquid flow path is not smooth, or no liquid is taken up it could be one or more of the following factors:

• Pump tubing that is crimped or bent does not give a smooth flow rate. Re-adjust the pump tubing over the rollers to straighten out any kinks.
• A pump speed which is too low causes strong pulsations or spurting sample flow. Adjust the flow rate until the sample uptake is smooth.
• Pump tubing that is flat or has lost its elasticity should be replaced with new tubing.

Tip If a low pump speed is required to restrict the uptake rate (for example, for some organic samples) select a smaller ID pump tubing to avoid strong pulsations yet still maintain a low uptake rate.

There will be a small degree of pulsing visible which is acceptable.

Air Leak

Check all tubing and connections in the sample introduction system for air leaks.

Instrument Calibration

If the instrument is reading on the edge of a peak (due to an instrument calibration error) the signal may be noisy. If this occurs perform an instrument calibration.

Aqueous Analysis After Non-aqueous Analysis

When performing aqueous analysis after running non-aqueous organic samples (or vice versa), a mutually miscible solvent must be used to ensure a thorough washout.

Traces of organic samples can remain in the spray chamber or torch. Rinse and if necessary, clean the spray chamber or torch when switching from organics to aqueous samples.

Partially Soluble Salts

Salts which are only sparingly soluble may partially precipitate when nebulized, causing a noisy signal. Converting the salt to a more soluble form may help reduce the noise. For example, add boric acid to fluoride solutions to create the more soluble borofluorides.

High Salt Content

Samples with a high salt content exhibit more noise than more dilute solutions. If better precision is required, dilute the sample.